1. Field of the Invention
The present invention relates to a process for the preparation of symmetrical diaryl-acetylenes from aryl bromides and acetylene in the presence of a palladium catalyst and a base, the acetylene being introduced into the liquid reaction mixture using a high intensity gas dispersion means.
Symmetrical diaryl-acetylenes are valuable intermediates. For instance, diaryl-acetylenes of the type which carry, on each of the aryl radicals, a carboxyl, carboxylic ester or carbonyl halide group or a group which can be converted into a carboxyl group, an ester group or a carbonyl halide group, are partially hydrogenated at the triple bond and the resulting bifunctional diaryl-olefin can be converted by polycondensation to give a high temperature-resistant material.
2. Description of the Related Art
Diaryl-acetylenes can be prepared, for example, by dehydrohalogenation of diaryl-halogenoalkenes or by two-fold dehydrohalogenation of vic- or gem- diaryl-dihalogenoalkanes using strong bases under relatively severe reaction conditions (Houben-Weyl, Methoden der Organischen Chemie, Volume V/2a (1977), p. 53 et seq.). These reactions are not suitable for the preparation of diaryl-acetylenes containing groups which are sensitive to bases. Furthermore, the halogen compounds used as starting material are often difficult to obtain.
Another method for the preparation of symmetrical diaryl-acetylenes is the Castro-Stephans reaction (loc. cit., p. 563 et seq.). In this reaction, copper acetylides are reacted with aryl iodides in boiling pyridine to give disubstituted acetylenes. This multistage process is confined to using the relatively expensive and often poorly accessible aryl iodides to achieve acceptable yields.
Moreover, diaryl-acetylenes can be prepared starting from aryl iodides or aryl bromides and aryl acetylenes using palladium catalysis (Houben-Weyl, volume XIII/9b (1984), p. 987 et seq.). However, the aryl acetylenes required for this preparation can only be obtained at great expense in multi-stage processes.
Furthermore, it is known that diaryl-acetylenes can be prepared by reacting acetylene with halogen compounds in the presence of bis(triphenylphosphine)palladium dichloride and copper(I) iodide in solution in diethylamine (Tetrahedron Lett. (1975), 4467). Suitable halogen compounds for this reaction are bromoalkenes or bromopyridine; in the benzene series, this reaction is limited to aryl iodides. The reaction is carried out by introducing acetylene through a delivery tube for 6 hours into the stirred solution of a batch of 10 mmol of iodobenzene.